A voltage controlled oscillator (VCO) is a component that can be used to translate DC voltage into a radio frequency (RF) voltage. The magnitude of the output signal is dependent on the design of the VCO circuit and the frequency of operation is determined by a resonator that provides an input signal. Clock generation and clock recovery circuits typically use VCOs within a phase locked loop (PLL) to either generate a clock from an external reference or from an incoming data stream. VCOs are therefore often critical to the performance of PLLs. In turn, PLLs are essential components in communication networking as the generated clock signal is typically used to either transmit or recover the underlying service information so that the information can be used for its intended purpose. PLLs are particularly important in wireless networks as they enable the communications equipment to quickly lock-on to the carrier frequency onto which communications are transmitted.
In this regard, the dynamic operating range and noise performance of a VCO may limit or affect the performance of the PLL itself. As an example, the operating frequency of a commercially available ceramic resonator-based VCO is typically limited to 3,000,000,000 Hertz (3 Giga Hz or 3 GHz) and usually has a temperature drift of more than 10,000,000 (10 Mega Hz or 10 MHz) over the temperature range of −40° C. to +85° C. The phase noise of the ceramic resonator-based oscillator is usually −120 dBc/Hz at 10 kHz for an operating frequency of 1 GHz (or 1,000 MHz). A surface acoustic wave (SAW) resonator-based oscillator typically offers −135 dBc/Hz at 10 KHz at an operating frequency of 622 MHz and −122 dBc/Hz at 10 KHz for an operating frequency of 2.5 GHz. The typical SAW resonator-based oscillator has a relatively low phase noise, but its performance is very poor over the operating temperature range and it offers a limited number of operating frequency selections.
FIG. 1 is an illustrative schematic diagram of a known oscillator. As FIG. 1 shows, a resonator 10, e.g., a ceramic resonator, is capacitively coupled through capacitor Cc 13 to the base of transistor 16. Feedback capacitor C1 18 is also coupled to the base of transistor 16 and to feedback capacitor C2 19, which is grounded. The values of capacitors C1 18 and C2 19 are preferably adjustable. The emitter terminal of transistor 16 is grounded through inductor Lc 23. The collector terminal of transistor 16 is biased through inductor L0 26 with DC voltage supply Vcc 29. A resistor R2 33 is coupled across the base of the transistor to an inductor L0 26. An additional resistor R1 35 is coupled to voltage supply Vcc and grounded through capacitor C0 37. In this arrangement the ratio of the resistors R2 33 and R1 35 are selected so as to provide temperature stabilization during operation. An output signal may be capacitively coupled from the collector at Vo1. The output signal at Vo1 provides better isolation but poor phase noise performance. For less isolation but better phase noise performance an output Vo2 may be capacitively coupled from the emitter of the transistor. In addition, the output signals Vo1 or Vo2 are non-sinusoidal as they include the fundamental frequency plus the harmonics. As previously discussed, the phase noise performance of oscillators of this type are typically −120 dBc/Hz at 10 kHz for an operating frequency of 1 GHz and the frequency drift is typically 10 MHz over −40° C. to +85° C.
Of utility then are resonator-based oscillators, e.g., VCOs, that provide ultra low noise and low thermal drift performance along with an extended frequency range of operation.